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Overview

Metallo-β-lactamases are known for the resistance of pathogenic bacterial strains to penicillins and other related antibiotics. This is caused by their serine active-site. The protein contains a zinc ion, along with 227 amino acid residues. What makes this protein unique, is the αββα sandwich. The internal molecular summary shows a 2-fold axis passing through the zinc ion. Harmless strains of Bacillus cereus were only known to produce two similar monomeric metallo-β-lactamases. A third metallo-β-lactamase was discovered and produced by Xanthomonas maltophilia, did not raise concerns in the medical community. Recent findings show that the resistance to carbapenems in an increasing number of clinically noxious strains is caused by the synthesis of Zn2+ containing β-lactamases.

Function

This protein catalyzes the hydrolysis of a large number of penicillins, cephalosporins, and almost all β-lactams. It hydrolyse carbapenems efficiently, while it usually escapes the activity of the active-site serine enzymes. This can be threatening to the way we fight diseases, because the gene is most likely to be plasmid-borne, which would make it easy to spread to other populations of pathogenic species.

Structure

The polypeptide chain is divided into two domains made up of beta-sheets and alpha-helices in the following order:β1 β2 β3 β4 β5 α1 β6 α2 β7 α3 and β8 β9 β10 β11 α4 β12 α5 going from N to C terminals. α2 has two glycine residues on 93 and 94, which causes a kink. The core is formed by two β-sheets surrounded by five helices. This is the , which is the keystone to the protein. The β-sheets have a twist which is typical to most proteins. They can be superimposed on β2 to β6 and β8 to β12 by a 2-fold rotation. The ββ sandwich fold is similar to DNase I and the folds have no similarity with any known metalloprotein structure. Including D-alanine-D-alanine carboxypeptidase, which also fights antibiotics including penicillin.

Active Site

The location of the active site is at the bottom of a groove between two beta-sheets. The consisting of His86, His88, and His149. The ion has a fourth ligand that consists of a water molecule. All four ligands are arranged in a tetrahedral shape. Seven out of the nine conserved residues are located in the active site: His88, Asp90, Leu114, His149, Gly179, Asn180, and His210. NMR studies have suggested that Cys168, which is replaced by a serine in X.maltophilia, can act as a potential fourth Zn2+ ligand, but the -S distance is too long for the metal ion ligation.

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